According to some of Dr Ashby's research (there's an archive of it on this site) apparently 19% is the critical threshold for FOC where it starts to dramatically increase penetration. So, maybe get an exact calculation, and try and bump it up that 1 or 2 %? It might be difficult, and I wouldn't do it if it affects your arrow flight too much, but it might be worth it on larger animals.
EDIT: Sorry, just looked at the chart, and 17-18% isn't too much different than 19% on his preliminary data at least, so it may not be worth the tuning. You're probably good to go, but the reports are recommended reading to say the least. I'm really enjoying them!